In addition to performing surgical operations on animal tissues by means of mechanical instruments such as scalpels or knives, surgery may also be performed by passing radio-frequency current through animal tissues. There are essentially four main surgical operations that can be electrically performed, depending on the voltage levels and the amount of power applied to the tissue. These operations are typically described as dessication, fulguration, cutting and cutting with hemostasis. Often, dessication and fulguration are described collectively as coagulation.
The high-frequency current used in performing electrosurgical operations is typically generated by means of a radio-frequency generator connected to a power amplifier. The output of the power amplifier is in turn connected to the tissue mass by means of two electrodes. Surgical operations are performed by means of an "active" electrode which introduces the radio-frequency current into the tissue mass.
Since, as mentioned above, electrosurgical effects are primarily dependent on the power and voltage applied, the active electrode typically has a small cross-section to concentrate the power and limit the surgical effects to a small, controlled area. A return path from the tissue mass to the generator for the radio-frequency current is provided by a "passive" or "patient" plate which has a large area to prevent electrosurgical effects from taking place at the current return location. Alternatively, a pair of active electrodes may be used in a "bipolar" mode in which the electrosurgical effects are confined to the sample of tissue between the two electrodes.
A dessication operation is performed by holding the active electrode in firm contact with the tissue. Radio-frequency current passes from the electrode directly into the tissue to produce heating of the tissue by electrical resistance heating. The heating effect destroys the tissue cells and produces an area of necrosis which spreads radially from the point of contact between the electrode and the tissue. Due to the nature of the cell destruction, the necrosis is usually deep.
Depending on the output characteristics of the electrosurgical generator, another surgical effect called fulguration may be obtained by varying the voltage and power applied by the electrosurgical generator. Although fulguration is often confused with dessication, it is a distinctly different operation. In particular, fulguration is typically performed in prior art devices with a waveform which has a high peak voltage but a low duty cycle. If an active electrode with this type of waveform is brought close to a tissue mass and if the peak voltage is sufficient to produce a radio-frequency arc, fulguration occurs at the point where the arc contacts the tissue. Due to the low duty cycle of the fulgurating waveform, the power per unit time applied to the tissue is low enough so that cutting effects due to explosive volatization of cell moisture are minimized. In effect, the radio-frequency arc coagulates the tissue in the immediate vicinity of the active electrode thereby allowing the operating surgeon to seal off blood vessels in the vicinity of the electrode. The fulgurating electrode never touches the surface of the tissue. In contrast to dessication, fulguration is a surface process and the area of necrosis is confined to the surface. Therefore, fulguration can be used where the tissue mass is very thin and the deep necrosis produced by a dessication operation would damage underlying organs and accordingly, is a very useful operation.
With different output characteristics of the electrosurgical generator, still another effect can be produced. Cutting occurs when sufficient power per unit time is delivered to the tissue to vaporize cell moisture. If the power applied is high enough a sufficient amount of steam is generated to form a layer of steam between the active electrode and the tissue. When the steam layer forms, a "plasma" consisting of highly ionized air and water molecules forms between the electrode and the tissue. If the electrosurgical generator can provide sufficient power, a radio-frequency electrical arc develops in the plasma. When this happens the current entering the tissue is limited to an area equal to the cross-sectional area of the arc where it contacts the tissue and thus the power density becomes extremely high at this point. As a result of the locally high power density the cell water volatizes into steam instantaneously and disrupts the tissue architecture--literally blowing the cells apart. New steam is thereby produced to maintain the steam layer between the electrode and the tissue. If the power density delivered to the tissue mass is sufficient, enough cells are destroyed to cause a cutting action to take place. A repetitive voltage waveform, such as a sinusoid, delivers a continuous succession of arcs and produces a cut with very little necrosis and little hemostasis.
It is also possible to achieve a combination of the above effects by varying the electrical waveform applied to the tissue. In particular, a combination of cutting and dessication (called cutting with hemostasis) can be produced by periodically interrupting the continuous sinusoidal voltage normally used to produce an electrosurgical cut. If the interruption is of sufficient duration, the ionized particles in the plasma located between the electrode and the tissue diffuse away, causing the plasma to collapse. When this happens the electrode comes in contact with the tissue momentarily until a new plasma layer is formed. During the time that the electrode is in contact with the tissue it dessicates the tissue thereby sealing off small blood vessels and other bleeders in the vicinity of the electrode.
The four above-described surgical operations thus require the electrosurgical generator to operate into a varying tissue impedance at various power levels. Although many prior art devices have produced satisfactory results with dessication and cutting operations, most prior art electrosurgical generators have failed to produce satisfactory fulguration. In particular, the electrical arcs produced by prior art generators operating in a fulguration mode are rather short in length and duration. As a result the active electrode must be moved very close to the tissue being fulgurized. If the active electrode actually touches the tissue during the fulgurization operation, tissue may cling to the electrode causing tissue damage and fouling of the electrode which then must be cleaned by the operating surgeon. Thus, the spark length of the prior art machines makes them highly unsatisfactory in many situations especially when the tissue being fulgurized is pulsating due to blood flow or in motion due to respiration. This problem is further exacerbated by the tendency of prior art machines to deliver a significant amount of power to the fulgurated area. In order to increase the arc length, many machines have increased the output fulgurating voltage and thus the output power. The high power input causes the the fulgurated tissue to pucker and swell, thereby increasing the probability of electrode contact.
In addition, prior art electrosurgical generators have not been able to satisfactorly fulgurate spongy or vascular tissue such as a spleen or liver. Since these organs virtually ooze blood through their vascular tissue structure it is very difficult to coagulate an incision to produce satisfactory hemostasis. The relatively high output power of the prior art machines actually initiates secondary bleeding under the area of eschar in this type of organ. In addition, if these machines are used on an organ for more than a short time, the high power produced may actually overheat an entire organ by electrical resistance heating to cause serious damage.
Due to the above shortcomings of the prior art, many surgeons have relied to a great extent upon an older electrosurgical generator which produces a fulgurating radio-frequency output by means of a spark gap. These devices are known as "Bovie devices" and typically produce a 12,000-14,000 volt peak-to-peak highly damped sinusoidal waveform when used during fulguration. Although they are relatively old devices they still produce the most satisfactory fulguration waveform. Even though these deivces operate better than most modern solid state devices they still produce fulgurating arcs which are short in length and duration. Also, these units are large and bulky and require continuous maintenance to replace the internal spark gaps. Because of these shortcomings, other surgeons have entirely foregone the fulgurization function of electrosurgical generators, preferring to rely on the machines only for cutting and dessication.